This invention, as was the parent, pertains to the joining of profile wall thermoplastic pipes including polyolefin, polyethylene, polyvinyl chloride, nylon, polybutylene, polypropylene and the like. These types of materials are gaining popularity in water, sewer, culverts and industrial piping because of their characteristics of being lightweight, corrosion resistance, strong and durable.
"Trenchless" rehabilitation of culverts, storm sewers, sanitary sewers and other underground pipes, by "slip lining" or "insert renewal" with thermoplastic pipes is gaining popularity and growing rapidly throughout the United States and other countries. In this process, a thermoplastic pipe or liner is inserted into an existing pipe or culvert without removal of the deteriorated pipe. The replacement pipe is pushed into or pulled through the existing culvert. In many cases, an existing pipeline can be rehabilitated for a fraction of the cost of replacement and with minimal inconvenience to the public.
Thermoplastic pipes, including polyethylene, are the preferred pipe material for many rehabilitation projects because of the price and the above-noted characteristics. Generally, thermoplastic pipe is manufactured in lengths that are sufficiently short to permit transportation and handling. In the field, where the pipe is to be installed, the short pipe sections must be connected to form a continuous pipe of a predetermined length appropriate for the application.
The joining or connecting of thermoplastic pipes can present many problems because of the variety of field conditions encountered and because of thermoplastic pipes chemical resistance, which, in many cases, makes such pipes impervious to glues or cements. Also, some thermoplastic pipes have a tendency to "creep," or move, when subjected to changing temperatures. Because most applications include exposure to such temperature changes, such movement or "creeping" limits the ability to use mechanical type joints such as threads.
In general, several methods exist to join thermoplastic pipe in the field. A first method is "butt fusion" as it is known in the art. This method involves the use of a butt fusion machine that includes line up equipment and a heat plate. The ends of two pipes to be joined are inserted into the line up equipment that aligns and advances the pipe ends toward one another as necessary. The two pipe ends are pressed against a heat plate that heats and softens the two pipe ends. The heat plate is then removed and the line up equipment advances the two pipes toward one another at a predetermined rate (depending on the size and thickness of the pipe walls) in order to fuse the pipe ends together. This type of butt fusion requires special fusion equipment that is expensive and not always available in the field and cannot be used with certain types of pipe.
Another method known in the art is the use of electrofusion collars or inserts. One type of electrofusion collar is shown in U.S. Pat. No. 4,530,521 to Nyffeler, et al. One type of electrofusion insert is shown in U.S. Pat. No. 3,768,841 to Byrne at al. These devices, as shown in the references, use a sleeve, collar or insert made of thermoplastic material and which either fits over or into the two pipes to be joined. The pipes, and the collar or insert are first heated to soften the thermoplastic material. If using a collar, the pipe ends are inserted into the collar and are thereby joined. If using an insert, the insert is inserted into each pipe end thereby joining the pipes. The heating can be performed with fusion equipment or the collar or insert can contain an electrical resistance element to provide the necessary heat to cause electrofusion welding of the pipes to the collar or insert.
These devices have various disadvantages, including the creation of interior obstructions or exterior protrusions that are not acceptable in many applications. For example, because the collar must be large enough to accept insertion of the pipe ends, the resulting joint does not have a flush exterior. Also, because the insert reduces the inside diameter of the pipes at the joint, the insert acts as an obstruction of flow through the pipe. This is unacceptable in most applications, including most trenchiess rehabilitation projects, because interior flow obstruction is not acceptable. Furthermore, exterior collars impede insertion of the replacement of the pipe during slip lining, as known in the art, and require the use of smaller diameter replacement pipes so that the collar can fit over the pipe and the entire coupling can still fit inside the existing pipe or culvert.
Another method of joining thermoplastic pipes is electrofusion rods or mesh as shown in U.S. Pat. No. 5,410,131 to Brunet et al. Although this device requires no collar or insert, the application requires substantial end pressure to join the two pipe ends. Such pressure is usually supplied by special line up equipment due to the weight of the pipes and this equipment is expensive and not always available or practical for use in field conditions.
Another method, hot air gun welding uses a welding rod of thermoplastic material fed through the nozzle of a hot air gun. The hot air gun applies heat to the ends of the pipe to be welded and melts the welding rod which is applied to a bevel cut between the two ends of the pipe to be joined. Line up equipment must be used, as with the butt fusion methods, and this method has not proven satisfactory in field conditions due to a lack of uniformity in the welding process.
As thermoplastic resin prices increase, plastic pipe manufacturers are constantly looking for manufacturing methods to make pipe lighter without losing physical strength. One type of thermoplastic pipe developed to address these concerns and is gaining popularity is "profile wall pipe" as it is known in the art. An example of this type of pipe is shown in U.S. Pat. No. 5,362,114 to Levingston. Profile wall pipe is thermoplastic pipe formed by extrusion to have an inner cylindrical wall, a generally concentric outer cylindrical wall and a helical rib between and connecting the inner wall and outer wall.
Profile wall pipe is lighter than solid pipe and is created with less material thereby reducing resin costs, but it maintains a high degree of strength. Because it is lighter than solid wall it generally has a competitive advantage over solid wall plastic pipe. For these and other reasons, profile wall pipe is popular in the industry. Unfortunately, typical methods of fusing thennoplastic pipe are unacceptable and do not work on profile wall pipe. This is due, in part because of the configuration of the end walls of such pipe which do not present a solid annular surface due to the presence of the helical rib contained in the pipe. Instead, the end walls of profile wall pipe comprise a thin inner wall, a thin outer wall and a apace between the inner and outer walls formed by the helical rib.
For example, butt fusion is very difficult on profile wall pipe because the pipe ends of profile wall pipe are not solid. The profile wall pipe ends have a thin inner wall, a thin outer wall and a "profile space"--between the inner and outer walls--the depth of which is equal to the distance between the end of the pipe and the helical rib which connects the inner and outer walls. The same problems that exist in joining solid wall thermoplastic pipes are multiplied in profile wall pipes because of their relatively thin inner and outer walls with a large profile space between the walls.
For example one manufacturer produces a polyethylene 10" inch inside diameter profile wall pipe that has inner and outer wall thicknesses of 0.065" inches to 0.079" inches. This particular pipe has a profile distance over 1 inch between the inner and outer walls with an outside diameter of up to approximately 11.20" inches. A 36" inch inside diameter profile wall pipe from the same manufacturer has inner and outer wall thicknesses of approximately 0.195" to 0.228" inches and an outside diameter of approximately 40.65" inches giving a profile distance between the inner and outer walls over 4 inches.
Butt fusion of profile wall pipes is very difficult due to the thin wall thickness compared to the overall diameter. Setting the correct hydraulic pressure on a butt fusion machine for such thin walls and large diameters would result in extremely slow fusion machine carriage movement and potential cooling of the thermoplastic prior to fusion joining. This results in a failed weld or "cold joint" as known in the art. Rods or mesh alone also will not work on profile wall pipe because profile wall pipe does not have solid flat pipe end surfaces which are required in those methods.
For example, U.S. Pat. No. 5,494,318 Butts et al. discloses a secondary containment piping system composed of a plurality of modules of concentric pipe. However, the invention of Butts would not work with profile wall pipe because when joining concentric pipes together using the apparatus of Butts, one must use line-up equipment. Dual containment pipe, unlike profile wall pipe is essentially two separate thermoplastic wall pipes with walls of sufficient thickness that butt fusion is a readily acceptable means of joining. In addition, as disclosed in Butts, a welding rod is placed between the solid ends of the pipe members to be joined, pressure is applied and maintained while an electric current is passed through the wires causing the melting of the core of the welding rod and the adjacent portions of the members. The pressure is maintained after the current is discontinued until the members are fused together. Column 1, lines 35-45. Specifically, Butts discloses positioning an annulus of welding rod between the ends to be joined and butting the ends together with the appropriate maintenance of pressure while an electric current is supplied for a sufficient time to cause fusion of the members and the welding rod. This will simply not work in profile wall pipe for the reasons stated above. Moreover, in order for this type of pipe fusion to work, the wall thickness of the inner and outer walls would have to be increased to such an extent that the advantages of using profile wall pipe are lost. Increased material and weight of the pipes reduces the effect of using profile wall pipe. Also, dual containment pipe is to satisfactory for the applications for which profile wall pipe is usually used for the reasons noted above.
Butts also discloses the use of a "fusion ring" for coupling sections of dual containment pipe. However, Butts requires the use of a welding rod element internally within the fusion ring with the ends of the pipe being received in opposite sides of the ring and held in position while the welding rod is heated and fuses with the ends of the pipe to form the complete joint. The fusion ring is formed of a welding rod and a collar surrounding the welding rod, the collar having an inner inside diameter substantially equal to the outside diameter of the sections to be joined. The fusion ring is positioned between two sections of pipe as in a socket fitting and the two sections are then welded together while pressure is applied to push the pipes together.
This fusion ring of Butts et al. is not acceptable for profile wall pipe and it continues to cause interior and exterior protrusions which can impede flow and impede the use of the pipe as replacement culvert systems. Moreover, profile wall pipe is different from dual containment pipe and does not have the advantages of profile wall pipe. Dual containment pipe is essentially two solid wall pipes concentric with one another. Thus, those applications that work with solid wall pipe will work with dual containment pipe but not with profile wall pipe. Therefore, the invention disclosed in Butts would not be applicable to nor functional with profile wall pipe to obtain the advantages provided by the present invention.
For example, profile wall pipe, unlike dual containment pipe, is not two separate concentric pipes. In fact, profile wall pipe is a single pipe, which, as explained above, is extruded and used for its lightweight replacement capabilities. However, the inner and outer walls of the profile wall pipe are so thin that the welding rod of Butts would not work on profile wall pipe and, for the reasons discussed above, one cannot butt fuse profile wall pipe due to the complexity of the process and the length of time and pressure that would be required to cause the pieces of pipe to fuse together. Moreover, in order to prevent protrusion of the welding rod into and out of the pipe so as to maintain flush surfaces, one would be required to use such a thin welding rod that it would not be sufficient to carry enough current and make a sufficiently consistent weld.
Another method, hot gas welding, when used with profile wall pipes will not achieve a strong, uniform joint even with the most experienced welders. Prior art electrofusion collars or inserts would be very bulky and either seriously interrupt interior flows or have a very large collar on the outside of the pipe, making the pipe joint unsuitable for slip lining or pipe rehabilitation applications.
One method for joining profile wall pipes is shown in U.S. Pat. No. 5,362,114 to Levingston. As shown therein, profile wall pipe is joined by threaded engagement. Shaving reveals the threads, formed by the helical rib during creation of the pipe, along the inner wall of one pipe section and the outer wall of another pipe section. This allows the two pipe sections to be threaded together. However, this method of joining is not sealed and requires the use of sealants or gaskets to make the joint liquid tight or leak-proof and does not provide the beneficial characteristics of a fusion welded joint. For example, a fusion weld is also air tight whereas a threaded joint, even with sealants, is not acceptable for gas pipelines which require airtight seals.
Thus, prior art collars or inserts result in interior flow obstructions and or collars or exterior protrusions which would prevent the pipe insertion in trenchless applications. Prior art welding rods or mesh require line up equipment to push the ends of the pipes together in solid wall pipe and would not be acceptable for the joining of profile wall pipes. Prior art threaded methods for joining profile wall pipes are not leak proof without sealants or gaskets. Furthermore, specifications in many applications call for a leak-proof joint that has a flush interior and exterior pipe surface and in trenchiess applications is strong enough to withstand pulling or pushing the pipe through an existing pipeline. The above type methods do not satisfy these specifications.